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WASHINGTON — A military appeals court sided with the Pentagon on Monday, overruling a judge who threw out terrorism charges against a Guantanamo Bay detainee.

The U.S. Court of Military Commission Review ruled that a military court set up by the Bush administration was the proper venue for deciding whether Canadian citizen Omar Khadr is an "unlawful enemy combatant" and trying him on terrorism charges.

The ruling reverses a military judge's June 4 ruling that the tribunal system created by Congress did not have authority to try detainees unless they were first determined to be unlawful enemy combatants.

That ruling threatened to force the Pentagon to start over with tribunals for a number of detainees. Pentagon officials argued that the June 4 ruling was just a matter of semantics and was insufficient to dismiss the case.

Monday's decision, the first ever by the newly formed appeals court, agreed.

The appeals judges, who are military officers, said the trial judge "erred in ruling he lacked authority ... to determine whether Mr. Khadr is an 'unlawful enemy combatant' for purposes of establishing the military commission's initial jurisdiction to try him."

The court battle over the Khadr case represents the latest problem for the Bush administration's military commissions system, which exists outside the traditional military and civilian rules of justice. In 2006, the Supreme Court ruled that President Bush's plan to try Guantanamo Bay detainees in military commissions violates U.S. and international law.

The White House persuaded the then-Republican-controlled Congress to weigh in with a law to legitimize the commissions. That law now faces court challenges.

"We will proceed in the most expeditious manner to get military commission cases to trial," the Pentagon said in response to Monday's decision. "The timeline is up to the judge. He decides when we will be back in the courtroom. The court's ruling outlined what must be done to establish jurisdiction."

Khadr was captured when he was 15 and faces charges of murder, conspiracy, spying and supporting terrorism. He is charged with tossing a grenade that killed one U.S. soldier and injured another in Afghanistan in 2002.

He is the son of an alleged al-Qaida financier, and his family has received little sympathy in Canada, where it has been called the "First Family of Terrorism."